The present invention relates to a centrifugal compressor and particularly, but not exclusively, to a turbocharger centrifugal compressor.
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures), and are widely used in automobiles, powered boats and the like. A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. For instance, in a centripetal turbine the turbine housing defines an annular inlet passage way around the turbine wheel and generally cylindrical axial outlet passageway extending from the turbine wheel. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
All centrifugal compressors are subject to surge under low flow conditions, i.e. a condition of unstable operation in which large fluctuations in pressure and mass flow rate occur. In many cases, such as when the compressor is part of a turbocharger supplying air to a reciprocating internal combustion engine, the fluctuations in mass flow rate are unacceptable. Accordingly, much effort has been made to improve the surge margin of centrifugal compressors to improve the usable flow range of the compressor.
Map width enhanced compressors are known which seek to improve both surge and choke margins (the xe2x80x9cmapxe2x80x9d of a compressor is the term given to a plot of total pressure ratio across the compressor against mass flow through the compressor, the width of the map being defined between the surge and choke flow limits). In a conventional map width enhanced compressor a chamber adjacent the compressor inlet is separated from the outer periphery of the compressor wheel vanes by a wall provided with an annular slot, or a series of radial holes, which allows communication between the chamber and the compressor wheel. This communication increases the amount of gas reaching the compressor wheel during high flow and high RPM operation and re-circulates gas to the compressor inlet during low flow operation, stabilising the compressor at choke and surge speeds respectively.
While such measures have been shown to be effective the present invention seeks to improve the surge margin of compressors yet further.
According to the present invention there is provided a compressor comprising a housing defining an inlet and an outlet, and a compressor wheel mounted for rotation about an axis within a chamber defined by the housing between the inlet and the outlet, the front of the compressor wheel facing said inlet and a portion of the back of the compressor wheel being set into a recess defined by a wall of said housing, said compressor wheel having a plurality of blades extending from the front side of said compressor wheel, the outside diameter of the recess being greater than the outside diameter of the recessed portion of the compressor wheel defining an annular clearance gap around the recessed portion of the compressor wheel, wherein the outside diameter of the recess is at least 1.05 times the outside diameter of the recessed portion of the compressor wheel and/or the depth of the recess in the region of said gap is greater than 1.5 times the axial width of the recessed portion of the compressor wheel.
In accordance with the invention the recess can be enlarged either radially or axially (in the region of the compressor wheel blade tips), or both. The enlargement to the recess provides the site for formation of the vortex mentioned above. The typical enlargement of the diameter of the recess will be between 1.05 and 1.15 times the outside diameter of the recessed portion of the compressor wheel. Test have shown that improvements in the surge margin are evident for a recess at least 1.05 times the diameter of the recessed part of the compressor wheel (typically the wheel back plate) and increase as the recess enlargement increases. However, loss of performance offsets the gain in surge margin for recesses made much more than about 1.15 times the diameter of the wheel. The preferred enlargement range is 1.10 to 1.12 times the diameter of the wheel
In embodiments in which the depth of the recess is increased this can be achieved by machining a groove within the recess adjacent the periphery of the recessed part of the wheel.
Other preferred features of the invention will be apparent from the following description of various different embodiments of the invention.